1. Field of the Invention
The invention relates to a comb filter arrangement, for example for separating a color picture signal into a modulated chrominance subcarrier signal and into a luminance signal, in which arrangement signals of three picture lines, made simultaneously available by means of two successively arranged delay elements which have a delay time of two picture lines for
signals and one picture line for NTSC signals, are utilized for obtaining two comb filter functions, and in which arrangement the signal having the median instantaneous amplitude is constantly filtered out of the signals of the three picture lines and the two comb filter functions.
2. Description of the Related Art
Conventional color picture signals, i.e. particularly color television signals carry the chrominance information components incorporated in these signals as a chrominance subcarrier which is modulated in dependence upon these information components. When such color picture signals are displayed, it is a problem to separate this chrominance signal, comprising the chrominance information components as a modulated chrominance subcarrier, from the remainder of the color television signal, particularly from the luminance signal. Simple bandpass filters may be employed for this purpose but they do not manage to fully separate the spectrally interleaved components of the chrominance signal and the luminance signal. Further, comb filters are known with which a picture line is submitted to such a delay that this delayed signal is in anti-phase with the signal of another picture line as far as the chrominance subcarrier components are concerned. These two components are subtracted so that essentially the components of the chrominance subcarrier are left. A separation between the chrominance signal and the luminance signal is possible in this way.
Furthermore, there is an integrated circuit of the type TA 8748 N made by the firm of Toshiba. For NTSC signals two delay elements are provided externally, each element realizing a delay of one picture line. Two comb filter functions are gained from the undelayed and the two delayed signals. From the signals of the actual picture line and the output signals of the two delay elements as well as from the two comb filter functions, the selection of that one out of the five signals that has the middle amplitude is realized in the Toshiba circuit by means of two successively arranged median filters. The problem in this circuit arrangement is that there are different delay times for the five signals of which the signal which has been filtered out appears at the filter output. The reason is that two successively arranged median filters are provided and that some signals only pass through one of the two filters. The resultant different delay times of the signals have negative consequences for the filtering operation because the phase relationship is of extreme importance for the subsequent addition of the filtered chrominance signal to the luminance signal, since the desired effects of cancellation or addition are only produced in an undistorted form when the two signals have an exact phase position with respect to each other.